Phase shift masks are gaining attention as the next generation lithographic technique for microelectronic fabrication due to their capability to produce higher resolution images compared to the conventional binary photomasks. Among the several phase shifting schemes, the attenuating embedded phase shifter proposed by Burn J. Lin, Solid State Technology, January issue, page 43 (1992), the teaching of which is incorporated herein by reference, is gaining wider acceptance because of its ease of fabrication and the associated cost savings. There have been a number of variations associated with this scheme to improve the optical properties of the photomask, i.e. tunability of the optical transmission and resistance against photon irradiation and chemical treatments. 157 nm lithography is being considered as a post 193 nm lithography scheme. Currently, there are no mature APSM materials for 157 nm that exhibit appropriate optical properties, tunability, radiation and chemical durability, etch selectivity, low defects, and ease of manufacturability. Previously in U.S. application Ser. No. 09/793,646 filed on 26 Feb. 2001, we disclosed an APSM Material and process based on SiTiN,SiTiON system for 193 nm lithography—the teachings of which are incorporated herein by reference.
Herein we describe compositions of matter and methods of fabricating 157 nm APSM materials in particular a stacked bi-layer structure and methods of fabricating a phase shift photomask that has tunable optical transmission, coupled with stable optical properties during usage (photon exposure and chemical treatments) of the photomask, and a superior etch selectivity. The composition consists of SiMkOyNz materials with an etch stop layer, where element M represents a metal as described in the claims.